1. Field of the Invention
The present invention relates to a planarization method, and particularly to a method of planarization using interlayer dielectric and a buffer layer.
2. Description of the Prior Art
The starting material for a very large scale integration (VLSI) circuit is ideally flat. After a great number of processes required to fabricate the VLSI circuits are performed, an increasing nonplanar surface occurs as the process proceeds to the metallization stage. This nonplanarity generally results in the difficulty in transferring fine-line patterns over the wafer. FIG. 1 shows a cross-sectional view illustrative of a conventional planarization method using a borophosilicate glass (BPSG) layer 10 as interlayer dielectric (ILD), which serves as an interlayer between metal layers and the underlying transistors. In this conventional fabrication, a barrier layer 8 is firstly deposited over the transistors. This barrier layer 8 is used to block autodoping of Bs or Ps from the BPSG layer 10 above. Dashed line 12 denotes the surface after the initially deposited BPSG layer 10 is reflowed. A chemical-mechanical polishing (CMP) method is then utilized to achieve a global planarization over the substrate 6. Unfortunately, the polishing rate using the CMP over the BPSG layer 10 varies greatly according to nonuniform concentration distribution of Bs or Ps. More specifically, the polishing rate over the region having low concentration becomes large, and vice versa. Therefore, a nonplanar surface 14 is thus produced, thereby degrading the following metallization process.